The invention relates generally to mapping and ablating tissue, and more particularly, to an improved system and method for mapping and ablating cardiac tissue with the same electrode.
In many cases, damaged tissue interferes with the proper functioning of an organism. As one example, the sinus and AV nodes provide the electrical control signals that cause the correct movement of the heart in pumping the blood to the body. Damage to tissue between such nodes may cause the control signals to be disrupted resulting in cardiac arrhythmias.
While there are different treatments for cardiac arrhythmias, including the application of anti-arrhythmia drugs, ablation of such damaged tissue has been found in many cases to restore the correct operation of the heart. Such ablation may be performed during open heart surgery; however, a preferred therapeutic procedure is percutaneous ablation. In this procedure, a catheter is percutaneously introduced into the patient and directed through an artery to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. These catheter devices typically include five or six lumina used for different purposes such as carrying electrical wires, body fluids, or drugs.
A steerable electrophysiological ("EP") catheter may be used to position an electrode or electrodes for systematically scanning selected endocardial sites within the heart to detect the propagation of wave electrical impulses as they propagate across the heart during each contraction. Through the detection of irregular electrical impulses, the locations of damaged cells may be revealed. Once these damaged cells have been located, the physician may use an ablation procedure to destroy the damaged cells in an attempt to remove the depolarization wave obstruction and restore normal heart beat. Characteristics required of a percutaneous EP catheter include small size and flexibility.
Before the damaged tissue can be ablated, it must be located with some precision so that the ablation energy can be accurately directed. Ablation of undamaged tissue is undesirable as is only partial ablation of the damaged tissue. Numerous types of EP catheters have been developed for more accurately locating the damaged tissue. As indicated above, selected endocardial sites may be successively scanned or mapped to locate damaged tissue. A desirable characteristic of the mapping device is small size. The smaller the size, the higher the resolution that can be obtained in identifying damaged tissue. Larger scanning electrodes contact more surface area and therefore have lower resolution. Mapping electrodes of one millimeter in length have been disclosed.
Another characteristic desired of an EP catheter is the ability to perform both the mapping procedure and an ablation procedure without having to withdraw the catheter and re-introduce it or a different one into the patient. It is undesirable to have to replace a mapping catheter with a separate ablation catheter because of the increased trauma caused the patient and the difficulty of locating the replacement catheter exactly in the position of the replaced catheter. Thus, EP catheters have been developed that are capable of performing both mapping and ablation procedures without removing them from the patient once positioned.
A typical EP catheter includes an electrical connector at its proximal extremity that is coupled to the appropriate equipment to conduct the mapping and ablation procedures. For example, during mapping the electrical lines connected to the catheter electrodes will be connected through the connector to analysis equipment comprising computer controlled electrical signal sensors. During the ablation procedure, the electrical line or lines connected to one or more of the catheter electrodes, preferably the distal tip electrode, will be connected through the connector to a power box for supplying up to 100 watts of power at a frequency between 100 kHz to 30 mHz with variable or fixed impedance.
A typical EP catheter is shown in FIG. 1 and is described in further detail below. The catheter 10 includes an active electrode 12 at the distal tip 14 of the catheter tube 20 and ring electrodes 16 around the diameter of the tube spaced proximally from the distal electrode 12. The electrodes are connected to the proximal end of the catheter 18 with thin, flexible wires.
After the endocardium has been mapped and damaged tissue identified, ablation of the damaged tissue can be performed. Many EP catheters use radio frequency (RF) technology to destroy the damaged endocardial cells. The use of radio frequency energy for cardiac ablation has gained widespread acceptance and success in treating arrhythmias. Thermal tissue damage and ablation occur as a result of the application of radio frequency energy to cardiac tissue.
In practice, the catheter distal tip 14 is fitted with an electrode 12 used both for mapping and for emitting RF energy to destroy the target damaged cells. Such an active electrode is the source of an electrical or electromagnetic field that causes heating of the contacting and neighboring tissue. To be most effective, the electrode at the distal end of an RF ablation catheter is placed in intimate contact with the target endocardial tissue in order to avoid leaving a gap in which concentrated energy might boil the blood in the intracardial volume. However, even though the electrode is pressed into intimate contact with the endocardium, typically a portion of the electrode will be in contact with the blood. This is true in both the unipolar and bipolar approaches.
In the approach commonly referred to as "unipolar," a large surface area electrode is placed on the chest of the patient to serve as a return for completing the electrical ablation circuit with one of the catheter electrodes. In a bipolar approach, typically two electrodes on the catheter are used to complete the electrical circuit. This circuit may include the tip electrode and a band electrode located proximal to the tip. In the bipolar approach, the flux traveling between the two electrodes of the catheter enters the endocardium to cause ablation. In the bipolar system, as in the unipolar system, portions of the active electrodes typically are in contact with the blood so that boiling can occur if those electrodes reach an excessive temperature.
The temperature boundary between viable and non-viable tissue is approximately 48.degree. Centigrade. Tissue heated to a temperature above 48.degree. C. becomes non-viable and defines the ablation volume. For therapeutic effectiveness, the ablation volume must extend a few millimeters into the endocardium and must have a surface cross-section of at least a few millimeters square. The objective is to elevate the tissue temperature, generally at 37.degree. C., fairly uniformly to the ablation temperature above 48.degree. C., keeping the hottest tissue temperature below 100.degree. C. At approximately 100.degree. C. charring and boiling of the blood take place. Charring is particularly troublesome at the surface of the catheter electrode because the catheter must be removed and cleaned before the procedure can continue. Additionally, charring and boiling of the blood seriously modify the electrical conductivity of blood and tissue and cause an increase in the overall electrical impedance of the electrical heating circuit and a drop in the power delivery to the tissue. Too great a rise in impedance can result in sparking and thrombus formation within the heart, both of which are undesirable.
Even though no significant amount of heat is generated in the RF energy electrode itself, adjacent heated endocardial tissue heats the electrode via heat conduction through the tissue. As mentioned above, part of the active electrode will be in contact with the blood in the heart and if the electrode temperature exceeds 90-100.degree., it can result in blood boiling and clotting on the electrode. The application of RF energy must then be stopped. However, shutting the RF generator off due to the temperature rise may not allow sufficient time to complete the entire ablation procedure. Providing an ablation electrode capable of applying higher amounts of power for a longer period of time to ablate the damaged tissue to an acceptable depth is a goal of current ablation catheter electrode design. It has been found that higher power for longer time periods results in a higher probability of success of the ablation procedure.
Numerous studies have been performed on means to obtain greater ablation depths with an ablation electrode. For example, see Langberg, Lee, Chin, Rosenqvist, Radiofrequency Catheter Ablation: The Effect Of Electrode Size On Lesion Volume In Vivo, PACE, Oct. 1990, pages 1242-1248; Kuck and Schluter, Radiofrequency Catheter Ablation of Accessory Pathways, PACE, Vol. 15, Sep. 1992, pages 1380-1386; and Langberg, Gallagher, Strickberger, Amirana, Temperature-Guided Radiofrequency Catheter Ablation With Very Large Distal Electrodes, Circulation, Vol. 88, No. 1, July 1993, pgs 245-249. In each of these cases, the conclusion appears to favor a larger electrode; for example 8 mm in length. As a result, many EP catheter manufacturers have increased the size of the ablation electrode to obtain an increased ablation depth referred to in these papers. However, larger size electrodes are more difficult to steer into and position in the cardiac site and additionally, are not as desirable for mapping purposes. The electrode selected for ablation is usually mounted on the distal tip of the catheter and that location is excellent for mapping procedures. Increasing the size of that electrode not only makes it more difficult to steer into the mapping and ablation sites, but also provides lowered resolution in the mapping procedure as pointed out above. A larger electrode results in less sensitivity or resolution to determine the exact location of the aberrant tissue. It is thought by many skilled in the art that an eight mm length results in an electrode unsuitable for mapping purposes due to this lack of resolution.
However, reducing the size of the electrode is discussed in these references as resulting in a lower power handling capability with less desirable ablation patterns. For example, the Kuck and Schluter paper points out that the ablation success rate was significantly increased by use of a 4 mm length tip electrode over a 2 mm length electrode (page 1383, right column). Since that publication date of Kuck and Schluter, the Langberg papers discuss that 8 mm tips provide improved results. The inventors believe that these publications are representative of the current state of the art in which larger electrode lengths are used for ablation regardless of their degraded mapping performance.
The inventors believe that most, if not all, ablation electrodes currently in use are constructed of pure platinum or a platinum iridium alloy, typically "platinum 10 iridium" (90% platinum 10% iridium). Platinum has a relatively low thermal conductivity of approximately or less than: ##EQU1## where cal=calories, cm=centimeters, s=seconds, and C=Centigrade (ASM Metals Handbook Desk Ed., pgs. 1-52, 1985). While the platinum or platinum 10 iridium alloy is desirable because of it bio-compatibility, its low thermal conductivity decreases its ability to dissipate heat. Consequently, if the electrode is too small, it will provide poor dissipation of the heat accumulating in itself from its contact with the heated tissue. This poor heat dissipation may not be rapid enough and early termination of the ablation procedure would be required to avoid blood boiling and coagulation. If the ablation procedure is terminated too early, it will not be complete, the ablation procedure may not be successful, and must be repeated.
To avoid this undesirable situation, manufacturers are making platinum 10 iridium ablation electrodes larger so there will be greater surface area of the electrode in contact with the tissue to be ablated, larger amounts of power can be applied, and ablation will take a shorter time. Additionally, the larger size of the electrode will assist in heat dissipation. Disadvantages of larger electrodes as discussed above include lowered accuracy as a mapping electrode, less localization of the ablation energy, and difficulty in introducing and positioning the electrode in a patient. Not only do smaller electrodes map better, they also better focus the RF energy to the damaged tissue site thereby limiting ablation of undamaged tissue.
Some manufacturers provide electrodes that are smaller in size than the above-discussed ablation electrodes and are formed out of pure platinum, which has a higher thermal conductivity than platinum 10% iridium. However, to the inventors' knowledge, these electrodes are specified by the manufacturer for mapping purposes, not ablation. While pure platinum has a higher thermal conductivity than platinum 10 iridium, it is still relatively low when compared to other materials and heat build up in an electrode formed of pure platinum when used for ablation is still of substantial concern.
Another concern related to EP catheters is the ability to monitor the ablation electrode temperature. Knowing when that temperature is approaching and then finally reaching 90-100.degree. can greatly assist a physician in successful control over the procedure. However, the thermal conductivity of the materials surrounding the temperature sensor can affect its accuracy, especially in the case where the sensor is located internally in the ablation electrode.
It has been noted in some cases where the ablation electrode is formed of platinum 10 iridium and the sensor is located internal to the electrode as opposed to being mounted on the outer surface of the electrode, that the temperature at the outer surface of the electrode can be higher than the temperature at the sensor due to the slower thermal conduction of the material. There is thus a time lag and hence, the temperature indicated by the temperature sensor signal is suspect.
Hence, those skilled in the art have recognized the need for an electrode small enough to provide increased resolution when used for mapping purposes, yet large enough to perform a complete ablation procedure without exceeding temperature limits. Additionally, the electrode has to be formed of a bio-compatible material for the internal use with a patient. Furthermore, those skilled in the art have recognized the need to provide an ablation electrode small enough for increased maneuverability in the patient, that requires less ablation energy to ablate the target tissue, and that localizes the ablation energy to avoid ablating undamaged tissue. Additionally, an electrode design that enhances the ability to obtain more accurate temperature sensor signals is desirable. The present invention fulfills these needs and others.